Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 270924

Area Forecast Discussion
324 AM CST Sat Dec 27 2014

.SHORT TERM...(Today)

08z/2am surface analysis shows 1013mb low just north of KMLI, with
frontal boundary extending southwestward into Texas. Airmass ahead
of the front was initially quite dry as evidenced by the 00z KILX
upper air sounding: however, shallow low-level moisture has been on
the increase throughout the night.  Surface dewpoints have reached
the middle 40s across central Illinois, while a plume of 50s
dewpoints is noted further upstream pushing into Arkansas.  LAPS
soundings show this increasing moisture, with most local profiles
not saturated below 700mb.  As low pressure tracks further
northeastward into Wisconsin, it will pull the cold front into the
area later this morning, then to the Indiana border by mid to late
afternoon.  Continued moisture pooling along/ahead of the front will
result in increasingly deep moisture profiles, particularly across
the E/SE CWA this afternoon.  HRRR suggests only scattered light
showers across much of the area this morning, then shows more
widespread precipitation developing along/east of I-57 this
afternoon.  Think this solution makes sense as front gradually taps
into increasing amounts of Gulf moisture as it moves further east.
As a result, will carry chance PoPs across the western half of the
CWA throughout the day, with chance PoPs increasing to likely across
the E/SE this afternoon.  With cold front passing through the area,
high temperatures will be achieved this morning, followed by brisk
westerly winds and falling temperatures later in the day.  Due to
very mild current temps, have generally gone 2-4 degrees above MAV
guidance numbers.  End result will be highs in the upper 40s and
lower 50s, with temps dropping into the 30s along/west of I-55
during the afternoon.


.LONG TERM...(Tonight through Friday)

Cold front will slowly push eastward into the Ohio River Valley
tonight, with clouds and showers lingering across the E/SE through
at least the evening hours.  NAM is fastest with the departure of
the precip, while GFS tends to keep it around through the entire
night.  Based on slow forward movement of the front, have trended
toward the slower GFS.  As a result, will maintain low chance PoPs
along/south of I-70 through the overnight hours.  As temperatures
slowly fall into the lower to middle 30s across this area, any
lingering rain showers may mix with or change to light snow late.
Further northwest, skies will clear across the Illinois River
Valley, allowing lows to bottom out in the lower to middle 20s.

Sunshine and seasonably cool temps in the 30s will prevail Sunday
into Monday, before a surge of cold air arrives next week.  Models
have been consistently showing strong 1050mb high building southward
out of western Canada into the central CONUS by Tuesday/Wednesday.
Weak synoptic lift along the leading edge of the cold air may be
enough to trigger a few flurries across the area Monday night, but
the main story will be the colder temps.  High temps will remain in
the 20s Tuesday/Wednesday, while overnight lows dip into the single
digits and teens.  After that, Canadian high pressure will gradually
slide east of the region, allowing a southern stream system to
approach by the end of the week.  Model solutions vary with this
feature, so forecast confidence remains low at this point.  ECMWF
aggressively brings precip northward into south-central Illinois as
early as Thursday night, while the GFS is about 12-18 hours
slower.  Meanwhile, the GEM suggests the system may remain largely
southeast of the area across the Ohio River Valley.  Given the
uncertainties, have opted to go with a dry forecast Thursday night,
and to slow precip onset on Friday.  Have trimmed Friday PoPs to
just slight chance, with higher precip chances arriving Friday night
into Saturday.  Think slower/colder is the way to go, especially
given the Canadian airmass that will initially be in place.  End
result will be a chance of snow or a rain/snow mix mainly Friday
night into Saturday.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)

Low pressure has moved NE along the cold front this evening,
reaching just west of Macomb at 0530z. Clouds are still at VFR
levels for another hour or two across our forecast area. The low
pressure will continue to advance NE up the front, passing over
Lake Michigan by Saturday morning. As the low departs, a cold
front extending south of the low will advance across our forecast
area. Upstream observations and forecast soundings point toward MVFR
clouds progressing over the terminal sites after midnight, with IFR
clouds accompanying the low as it moves over NW IL between 07z
and 12z. An initial push of rain will mainly affect the eastern
terminals of SPI/DEC/CMI/BMI through 09z. Additional rains are
expected to develop behind the initial line of showers, helping
drop cloud heights to IFR for several hours around sunrise.
Precipitation should diminish by Saturday afternoon, as the cold
front advances SE across the area and dry air aloft begins to

Winds will remain steady from the south at 9-14kt the rest of
tonight. The KILX VAD wind profiler shows that the low level jet
has already developed in central IL, with 35-40kt of winds at 2k
FT. Therefore we continued to include LLWS conditions for the
first 4-5 hours of this TAF period. Winds may back to the
southeast in the vicinity of the low toward PIA later tonight, but
then shift around to the SW then W during the day on Saturday
behind the cold front. Colder air will arrive on the increasing
W winds, with speeds of 13-16kt expected during the afternoon.
Winds will eventually veer to the NW Sat evening, as they diminish
below 10kt.




LONG TERM...Barnes
AVIATION...Shimon is the U.S. government's official web portal to all federal, state and local government web resources and services.